The present invention relates to direct-digital frequency synthesizers and in particular to an apparatus and method for reducing the spurious signal components of the output signals of such direct-digital frequency synthesizers. A continued goal in the design of radio communications systems has been implementation of a simple, low cost frequency synthesizer that can provide large range discrete frequencies over a wide frequency spectrum. The direct-digital frequency synthesizer (DDS) is one such apparatus known in the prior art. The DDS can provide by external command any discrete and precise frequency within its range and resolution. Additionally, use of a DDS generally provides fast switching speeds, excellent: temperature and aging stability, and allows for continuous switching of the carrier signal. In light of such attributes a direct-digital frequency synthesizer is commonly used in phase and frequency modulation and frequency-hopping schemes, as well as spread-spectrum communications.
The output frequency of a DDS is determined by an input digital frequency control word. This frequency control word can be generated by a digital processor. Many modulation schemes can be implemented such as FSK, FM, PM or DPSK. Thus, for example, the output frequency can be frequency modulated according to the digitized information (FSK modulation). In addition to the FSK modulation, the window of modulation frequencies can "hop", thereby moving the window of modulated frequencies in the coded scheme to avoid jamming or decoding by unauthorized transceivers. It is the agility and the frequency resolution of the DDS that makes it superior to other synthesizers in such an application, and gives an unauthorized transceiver less time to lock on the correct code and them jam or decode it. Naturally, the authorized receiver must hop to the correct frequency to receive the modulated information. Typically, the receivers digital processor generates the correct digital control word for the receivers DDS and subsequent down conversion, demodulation and decoding.
The heart of the DDS is an accumulator. The accumulator is a digital integrator consisting of a latch and full adder with the output sum and the input digital control word as input signals to the adder. The frequency word determines the step size by which the accumulated sum is incremented. Each step represents a step or increment in phase. Thus, larger frequency words result in larger steps and phase. In addition, the rate of over-flow of the accumulator determines the output frequency. The output frequency is a fraction of the clock frequency; thus, its spurious performance has similarities to a fractional divider. However, some frequencies are not submultiples of the clock frequency, and accumulator over flow occurs with varying remainders left in the accumulator. The residue left in the accumulator varies the output periods such that a sequence of different output periods result. The average of this sequence of periods is the desired period and thus, determines the output frequency. However, since this sequence of periods is also different from an ideal repetition of the exact period, spurious line frequencies or "spurs" unique to each output frequency are created. Such spurs represent departure of the output waveform from a precisely periodic signal.
Prior art techniques of suppressing such spurs have typically involved use of a sine approximation technique. Such a sine approximation technique is typically based on the premise that the instantaneous error from an ideal sinusoidal frequency of constant period is reduced, thereby suppressing the spurious signal level. Alternate solutions are typically based upon feeding a value corresponding to a random sequence of values thereby randomly varying the spurs, thereby reducing such signals.
Unfortunately, the use of such prior art solutions has numerous limitations. Often close-in spurs, those near the desired frequency, are inadequately handled. Additional limitations exist due to the predetermined nature of the counter effect to the unwanted spur signal.
Accordingly, a need exists for an apparatus that is based upon DDS theory that yields a desired output signal of minimum distortion.